This invention relates to a cushioned earphone, and in particular to a cushioned active headset providing noise cancellation.
A conventional cushioned earphone, for example as known from U.S. Pat. No. 4,809,811, is shown in FIG. 1. of the accompanying drawings. The drive unit 10 within the earphone shell 12 is separated from the ear by means of the foam cushion 14. The cushion 14 serves two purposes.
The first is one of comfort, whereby the foam is compliant enough to partially mould around the irregularities of the ear and thereby spread the pressure of the earphone more or less evenly over the entire contact area. This avoids xe2x80x98hot spotsxe2x80x99 that can lead to soreness of the ear.
The second purpose of the foam is to allow the sound from the drive unit through to the ear more or less unimpeded whilst preventing it from leaking out to the surrounding space thereby reducing the sensitivity of the headset. This leakage takes place through the body of the foam itself as well as through any gaps that occur between the foam and the ear due to imperfect sealing.
These requirements are unfortunately contradictory. The best comfort and least leakage due to poor contact is obtained if the foam is deep and of low density so that it""s compliance is higher, but this allows more leakage through the foam and hence less sensitivity. Increasing the sensitivity by use of a denser foam not only reduces comfort but also forms more of a barrier between the drive unit and the ear.
There are ways to partially overcome these difficulties and one example is shown in FIG. 2 of the accompanying drawings. This approach has a cushion that is moulded with a thinner central region 14A so that there is less impediment to the sound passing from the drive unit 10 through to the eardrum, but there still remains the compromise between comfort and sensitivity in the choice of foam density.
Thus, with a conventional foam cushioned earphone, there is e acoustics of the headset when the earphone is pressed against the ear. Under these conditions the acoustics impedance of the foam increases, the leaks decrease and the volume between the drive unit and the ear canal also decreases. These factors cause the acoustic output of the earphone to increase. With a normal headset this merely causes frequency response variations (and a left/right imbalance if only one earphone is pressed against the ear), but with an active headset the results can be highly disadvantageous. With a virtual earth negative feedback type headset the rise in acoustic gain can lead to instability, whilst with a feedforward headset noise cancellation is severely degraded.
This difficulty in the choice of foam density occurs because of the inherent characteristics of conventional foams. As the material is compressed in one direction its tendency is to expand in the perpendicular directions and vice versa, maintaining more or less a constant volume. Thus if an object presses into a sheet of foam the thickness directly below the depression is reduced and therefore the region under the depression expands outwards. More importantly; however, the surface of the foam has been stretched in two dimensions over a fairly wide area in order to create the depression and the effect of this is for the thickness of the foam away from the immediate area of the depression to decrease, thus pulling the surface of the foam away from the object. In the case of a protrusion from a surface, as in the case of irregularities in the shape of an ear pressing into earphone foam, the result is to leave air gaps around the protrusion where sound can leak through. This effect is demonstrated in FIG. 3 of the accompanying drawings, wherein a typical air gap is referenced 15.